1. Field of the Invention
The present invention relates to a magnetic bearing control device and an exhaust pump having the control device, and particularly to a technology for identifying a movable range of an eddy current gap sensor configuring a magnetic bearing and a center of the movable range and lifting a rotor shaft and other controlled shafts, which are lifted by the magnetic bearing, at a central position of a protective bearing.
2. Description of the Related Art
Conventional exhaust pumps that are used as gas exhaust means of process chambers or other closed chambers in semiconductor manufacturing devices, flat panel display manufacturing devices, and solar panel manufacturing devices adopt magnetic bearings for the purpose of lifting and supporting a rotating shaft of a rotor (referred to as “rotor shaft” hereinafter) that is rotated when discharging gas. This type of magnetic bearings is disclosed in, for example, Japanese Patent Application Publication No. 2006-83924.
The magnetic bearing disclosed in Japanese Patent Application Publication No. 2006-83924 is bearing means for lifting and supporting a rotor shaft (113) of a turbomolecular pump (100) known as an exhaust pump, the magnetic bearing having eddy current gap sensors (107A, 107B) and electromagnets (104X+, 104X−) on an X-axis of an XY coordinate system and a control device (200) for controlling excitation currents of the X-axis electromagnets (104X+, 104X−). Although not shown, the magnetic bearing also has eddy current gap sensors and electromagnets on a Y-axis of the XY coordinate system.
The turbomolecular pump (100) of Japanese Patent Application Publication No. 2006-83924 is provided with a protective bearing (120) as an auxiliary device of the magnetic bearing. The protective bearing (120) functions to receive and stop an abnormal rotation of a rotor shaft (104) that occurs when the function of the control device (200) for lifting and supporting the rotor shaft (113) is disabled.
Incidentally, when the center of the protective bearing (120) and the rotation center of the rotor shaft (113) are not aligned, the rotor shaft (113) and the protective bearing (120) easily come into contact with each other even when the rotation of the rotor shaft (113) is normal. Therefore, initial adjustment is performed upon factory shipment of the turbomolecular pump (100) so that the rotor shaft (113) rotates about the center of the protective bearing (120).
The initial adjustment is performed according to the flowchart shown in FIG. 4 of the present application. The procedures of the initial adjustment are described hereinafter in accordance with the flowchart shown in FIG. 4.
The flowchart of FIG. 4 is started by, for example, pressing an initial adjustment start button, not shown, of the control device (200). Once the flowchart is started, the magnetic bearing control device (200) starts controlling the excitation currents of the X-axis electromagnets (104X+, 104X−) and the Y-axis electromagnets, not shown, while these excitation currents are ON (applied) (step 201).
Next, the control device (200) pulls the rotor shaft (113) in a +X direction using the +X direction electromagnet (104X+) (step 202). As soon as the pulled rotor shaft (113) comes into contact with an inner ring of the protective bearing (120), detected value of the eddy current gap sensor (107A) in a +X-axis direction and of the eddy current gap sensor (107B) in a −X-axis direction are read, and a +X-axis direction movable limit position of the rotor shaft (113) is identified based on the read detected value (step 203). With the same principle, the control device (200) identifies a −X-axis direction movable limit position of the rotor shaft (113) (steps 204, 205).
The control device (200) thereafter calculates and identifies a midpoint between the +X-axis direction movable limit position and the −X-axis direction movable limit position that are identified as described above, as the center of an X-axis movable range of the eddy current gap sensor, which is, in other words, the center of the X-axis protective bearing (step 206). When the center of the X-axis protective bearing on cannot be calculated and identified, the process returns to step 202 to reattempt the calculation and identification of the center of the X-axis protective bearing (No in step 207). When, on the other hand, the center of the X-axis protective bearing is calculated and identified, the excitation currents of the X-axis electromagnets (104X+, 104X−) are adjusted such that the rotor shaft (113) rotates around the identified center of the X-axis protective bearing (step 208, Yes in step 207).
Subsequently, with the principle same as the one of the abovementioned method of calculating and identifying the center of the X-axis protective bearing, the control device (200) calculates and identifies the center of the Y-axis protective bearing (the center of the magnetic bearing on the Y-axis) (steps 209 to 214), and adjusts the excitation currents of the Y-axis electromagnets, not shown, so that the rotor shaft (113) rotates around the identified center of the Y-axis protective bearing (step 215).
FIG. 5A of the present application shows a state in which the center (geometrical/mechanical center) of the protective bearing (120) coincides with the center (electrical center) of the X-axis movable range of each X-axis eddy current gap sensor and in which the center of the protective bearing (120) coincides with the center of each Y-axis movable range of the Y-axis eddy current gap sensor, when the conventional initial adjustment is performed according to the flowchart of FIG. 4. In such a state, the initial adjustment can be ended normally. In FIG. 5A of the present application, the X-axis eddy current gap sensor (107B) described above is illustrated as “+X sensor, −X sensor,” and the Y-axis eddy current gap sensor, not shown, as “+Y sensor, −Y sensor.”
However, in some cases the center of the protective bearing (120) does not coincide with the center of the X-axis movable range of each X-axis eddy current gap sensor as shown in FIG. 5B of the present application, due to attachment position errors of the eddy current gap sensors (+X sensor, −X sensor) or an attachment position error or dimensional tolerance of the protective bearing (120). In such a case, the conventional initial adjustment according to the flowchart of FIG. 4 causes a so-called oscillation phenomenon in which, when the rotor shaft (130) is pulled in the +X direction in step 202, the rotor shaft (130) reciprocates a little at a time along an arc surface of the inner ring of the protective bearing (120).
This oscillation phenomenon is due to the fact that the rotor shaft (130) is moved along the arc surface of the inner ring of the protective bearing (120) by a force component acting in an arc surface tangential direction of the inner ring, the component being a component of force pulling the rotor shaft (130) in the +X direction, and the fact that, since the position of the rotor shaft (130) in the Y-axis direction changes due to its movement, the excitation current for restoring the change acts on the electromagnet in the Y-axis direction, which is not shown. Note that the same oscillation phenomenon occurs when the rotor shaft (130) is moved in the −X direction in step 204 or when the rotor shaft (130) is pulled in the +Y direction or the −Y direction in step 209 or 211.
The occurrence of the oscillation phenomenon of the rotor shaft (130) described above cannot identify the X-axis movable range or the center thereof or lift the rotor shaft at the central position of the protective bearing. For this reason, shipment of the exhaust pump needs to be stopped due to poor initial adjustment.